Organic semiconductors are now frequently used in a number of optical devices such as in organic light emitting diodes (“OLEDs”) as disclosed in WO 90/13148, photovoltaic devices as disclosed in WO 96/16449 and photodetectors as disclosed in U.S. Pat. No. 5,523,555.
A typical OLED comprises a substrate, on which is supported an anode (commonly indium tin oxide or “ITO”), a cathode and an organic electroluminescent layer between the anode and cathode. In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an exciton which then undergoes radiative decay to give light. Other layers may be present in the OLED, for example a layer of organic hole injection material such as poly(ethylene dioxy thiophene)/polystyrene sulfonate (PEDT/PSS) may be provided between the anode and the organic electroluminescent layer to assist injection of holes from the anode to the organic electroluminescent layer.
Various classes of organic light emitting materials are known, in particular: polymers such as poly(p-phenylenevinylene) (as disclosed in WO 90/13148), polyfluorenes and polyphenylenes; the class of materials known as small molecule materials such as tris-(8-hydroxyquinoline)aluminium (“Alq3”) as disclosed in U.S. Pat. No. 4,539,507; and the class of materials known as dendrimers as disclosed in WO 99/21935. These materials electroluminesce by radiative decay of singlet excitons (i.e. fluorescence) however spin statistics dictate that up to 75% of excitons are triplet excitons which undergo non-radiative decay, i.e. quantum efficiency may be as low as 25% for fluorescent OLEDs—see, for example, Chem. Phys. Lett., 1993, 210, 61, Nature (London), 2001, 409, 494, Synth. Met, 2002, 125, 55 and references therein.
Accordingly, considerable effort has been directed towards producing luminescence from triplet excitons (phosphorescence) by utilizing spin-orbit coupling effects in metal complexes that enable triplet excitons to undergo radiative decay. The metal complex is doped into a host material from which it receives charge and/or triplet excitons. Examples of complexes investigated for this purpose include lanthanide metal chelates [Adv. Mater., 1999, 11, 1349], a platinum (II) porphyrin [Nature (London), 1998, 395, 151] and tris(phenylpyridine) iridium (III) (hereinafter “Ir(ppy)3”) [Appl. Phys. Lett., 1999, 75, 4; Appl. Phys. Lett., 2000, 77, 904]. Fuller reviews of such complexes may be found in Pure Appl. Chem., 1999, 71, 2095, Materials Science & Engineering, R: Reports (2002), R39(5-6), 143-222 and Polymeric Materials Science and Engineering (2000), 83, 202-203.
Prior art phosphorescent OLEDs often comprise charge transporting and/or charge blocking layers used in conjunction with the electroluminescent layer in order to maximize device efficiency. The charge transporting/blocking layers and the electroluminescent layers are typically formed by vacuum evaporation of the appropriate materials in sequence.
Deposition of materials from solution, for example by spin-coating or inkjet printing, offers advantages over vacuum deposition such as simplified processing. However, solution deposition of multiple layers is complicated by the tendency of initially cast or deposited layers to dissolve in the solvents used for succeeding layers. Thus, solution deposition will commonly be employed for one layer only. For example, U.S. 2002/096995 discloses spin-coating of an electroluminescent layer of polyvinylcarbazole (hereinafter “PVK”) host doped with Ir(ppy)3 emitter onto a substrate of ITO and PEDT/PSS. A layer of electron transporting material is then deposited by vacuum evaporation.
One solution to this problem is disclosed in JP 2003-077673 wherein a hole transporting layer of PVK is formed by spin-coating from 1,2-dichloroethane solution followed by formation of an electroluminescent layer of 9,9-dioctylfluorene host/Ir(ppy)3 emitter by spin-coating from xylene solution. The formation of this bilayer by solution processing is possible due to the low solubility of PVK in the xylene solvent used for the electroluminescent layer. This approach is limited in that the material used for the first layer can only be selected from those that do not dissolve in the solvent used for the second layer.
Another solution to this problem is disclosed in JP 2002-050482 wherein a first layer of insoluble poly(phenylenevinylene) (hereinafter “PPV”) is formed by deposition by spin-coating of a soluble precursor compound, followed by thermal conversion of the precursor to insoluble PPV. A layer of PVK host/Ir(ppy)3 guest is then deposited onto the insoluble PPV layer. Again, this approach is limited in that it is only applicable to insoluble compounds that have a soluble precursor form. Furthermore, the thermal conversion required by these precursors requires forcing conditions and generates corrosive by-products that may harm the performance of the finished device.